Antenna deployed from aircraft to contact a body of water for length reduction

ABSTRACT

The method and apparatus is disclosed for radiating lowfrequency radio waves. In particular, an extended length of a vertical radiator or antenna is suspended from a suitable aircraft, such as a helicopter, to extend a distance corresponding to approximately one-fourth of the wavelength of the radio wave to be radiated, and to contact sea water through a low-resistance connection. In this manner, a ground plane is formed for the antenna so that the physical length of the antenna may be reduced by approximately one-half and still be electrically one-half wavelength long as needed for efficient operation at resonance. In an illustrative embodiment of this invention, the suitable low-resistance connection may be effected by a drogue, which is attached to the lower end of the antenna to make a low-resistance connection between the antenna and the sea water. Illustratively, the drogue includes a surface area of an electrically conductive material, sufficiently large to make the low-resistance connection needed to conduct the large currents which flow at this point in the antenna. In operation, the aircraft takes off, and as it ascends, the antenna is unwound from a reel to provide the desired length dependent upon the frequency (wavelength) of the radio wave to be propagated. It may be understood that the length of the antenna is dependent upon the height of the aircraft. Further, a counterpoise may be extended from the aircraft to reduce the length of the vertical antenna required and also to reduce the voltage gradients established about the aircraft.

United States Patent [191 Tharp 5] 1 July 9, 1974 ANTENNA DEPLOYED FROM AIRCRAFT TO CONTACT A BODY OF WATER/FOR LENGTH REDUCTION Nelson B. Tharp, Ellicott City, Md.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Dec. 12,1972 [21] Appl. No.: 314,486

[75] Inventor:

[52] US. Cl...,. 343/705, 343/707, 343/719 Primary Examiner-Eli Lieberman Attorney, Agent, or FirmJ. B. l-linson 57 ABSTRACT The method and apparatus is disclosed for radiating low-frequency radio waves. In particular, an extended length of a vertical radiator or antenna is suspended from a suitable aircraft, such asa helicopter, to extend a distance corresponding to approximately one-fourth of the wavelength of the radiowave to be radiated, and to contact sea water through a low-resistance connection. In this manner, a ground plane is formed for the antenna so that the physical length of the antenna may be reduced by approximately one-half and still be electrically one-half wavelength long as needed for efficient operation at resonance; In an illustrative embodimentof this invention, the suitable low-resistance connection may be effected by a drogue, which is attached to the lower end of the antenna to make a lowresistance connection between the antenna and the sea water. Illustratively, the drogue includes a surface area of an electrically conductive material, sufficiently large to make the low-resistance connection needed to conduct the large currents which flow at this point in the antenna. In operation, the aircraft takes off, and as it ascends, the antenna is unwound from a reel to provide the desired length dependent upon the frequency (wavelengthlof the radio waveto be propagated. It may be understood that the length of the antenna is dependent upon the height of the aircraft. Further, a counterpoise may be extended from the aircraft to reduce the length of the vertical antenna required and also to reduce the'voltage gradients established about the aircraft.

8 Claims, 6 Drawing Figures PATENTEBJUL y V V 3.823.402 SHEEI 10? 2 PATENTED L 91974 SHEET 20F 2 DROGUE MIRROR IMAGE 0F ANTENNA FIG. 3

BACKGROUND OF THE INVENTION radiating radio wavesin the low frequency and the very low frequencywave range.

'2. Description of the Prior Art In the propagationoflow-frequency radio waves,-

there are difficult engineering problems in establishing a suitable antenna or radiating element. As the frequency of the wave to'be propagated decreases, the wavelength increases, thereby requiring, generally, a larger antenna structure to propagate efficiently the radio wave therefrom. In the low-frequency band of 30-300 KHz and in the very low frequency band of 3-30 KHz, the prior art has not suggested an efficient antenna for the generation of such radio waves. An efficient radiating element would be a vertical radiator whose length is physically adjusted or tuned electrically to be one-half wavelength long at the operating frequency of the radio generator. At; the frequencies under consideration, the length of the antenna becomes excessive so as to make the antenna impractical to support from a ground location. For example, if the operating frequency of the radio wave to be propagated is 22 KHZ, the length of a half-waveIength-long antenna would be in the order of 22,000feet. It is apparent that it would be difficult, if not impossible, to support an antenna of such lengthfrom the ground.

In the prior. art, the length of antennas suitable for propagating low-frequency waves has been reduced'by providing an umbrella configuration including a central vertical element supported upon the ground and having a plurality of elements extending from the top end portion in the form of an umbrella. In order to compensate for the lack of physical length, an inductance is connected in series between the central element and ground. However, such an inductance does present a resistance which introduces inherent losses in the antennas and large voltages are generated across the inductance presenting insulation problems. As aresult, additional power is required of the radio generator to compensate for the losses in efficiency due to the intro duction of such inductance.

SUMMARY OF THE INVENTION It is thereforean object of this invention to provide a new and improved method and apparatus for radiating low-frequency waves more efficiently with apparatus of less complexity and cost.

This and other objects are met in accordance with the teachings of this invention by providing an antenna which is suspended at its top-most end from a suitable aircraft, such as a helicopter. with its lower-most end contacting sea water. In particular. a suitable low resistance connection is formed between the lower end of. the antenna and sea water. whereby the sea water provides a ground plane for the antenna. As a result, the length of the antenna may be decreased signifn cantly; instead of requiring an antenna length of onehalf wavelength of the operating frequency of the radio wave to be propagated. a ground plane is provided by the sea water in accordance with teachings of this invention, to permit the length of the antenna to be reduced by one-half to one-fourth'wavelength of the op- I crating wavelength of the signal to be propagated.

In a further aspect of this invention, the suitable lowresistanceconnection between the lower-most end of the antenna and the sea water is provided by a drogue which is constructed to provide a large surface area of a suitable electrically conductive material whereby a low-resistance connection is made between the an tenna and the sea water, and the high currents established in the lower most end of the antenna are conducted to'the seawater. In one illustrative embodiment of this invention, the drogue may include at its lower end an opening which maybe selectively closed and opened to permit sea water tofenter the drogue, to thereby increase its weight and stability. Thus, the drogue may be easily carried empty by the aircraft and lowered into the water, at which time water is introducedinto the drogue. -When it is desired to remove the drogue from the sea water, the opening may be opened to permit thereby the release of the water and to lighten the drogue.

In still another featureof this invention, a counterpoise is deployed from the aircraft, whereby the potential gradients established about the aircraft and possible interference with the radio transmission are reduced. lllustratively, a suitable conductive sleeve of increased diameter with respect to the diameter of the counterpoise maybe disposed about the end portion of the counterpoiseto reduce the voltage gradient and the tendency to produce corona as the power is increased.

, BRIEF DESCRIPTION-OF THE DRAWINGS These and other objects and advantages of the present invention will become moreapparent-by referring to the following detailed description and accompanying drawings, in which:

FIG. 1 shows the deployment of an antenna system from a suitable aircraft in accordance with the teachings of this invention; V l

FIGS. 2A, 2B and 2C show schematically the various possible connections of the radio generator with respect to the aircraft housing;' i it FIG. 3 is a graphical representation of the magnitudes of the voltage and current distribution about the antenna of this invention; and

FIG. 4 shows a side, broken-away view of the drogue connected to the lower end of the antenna as shown in FIGS. 1, 2A. 2B and 2C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With regard to FIG. 1, there is shown an antenna element l8, suspended vertically from a suitable aircraft, such as a helicopter 10. The antenna 18 is of a suitable circular configuration and construction to be wound and unwound from a winch 14, carried aloft by the helicopter 10. Further, a suitable motor 12 is selectively controlled to wind and unwind the winch 14.

A drogue 20 is connected to the lower-most end of the antenna 18, whereby an efficient low-resistance connection is made between the antenna 18 and the sea water. generally indicated by the number 21. The antenna 18 is made of an electrically conductive material and is of sufficient diameter to withstand voltage gradithrough the antenna 18. Further, the antenna 18 should be mechanically strong so as to support its own weight and, as mentioned above, should be of suitable configuration such as circular to permit winding and unwinding from the winch 14. A circular configuration of the antenna reduces the wind resistance upon the antenna 18, as it is moved through the air by the helicopter 10.

In one illustrative embodiment, the antenna 18 has a diameter in the range of /a to /2 inch, with a suitable core about which is wound strands of an electrically conductive material, such as copper or aluminum. The core is made of a suitable material to add mechanical strength to the antenna 18, such as steel or a plastic such as fiberglass to reduce the, weight of the antenna 7 In accordance with teachings of this invention, the length of the antenna 18 may be reduced substantially by using the. sea water 21 as the ground plane for the antenna. More specifically, the drogue 20 provides a low-resistance connection between the lower-most end 'of the antenna 18 and the sea water 21. With regard to FIG. 3, there is shown the voltage and current distribution of an antenna in accordance with teachings of this invention. lnparticular, the length of the antenna 18 is determined to be approximately one-fourth of the wavelength of the operating frequency of the radio wave to be propagated. Normally, to provide a resonant antenna, the antenna will require a length of onehalf of the wavelength of theoperating frequency, however, the use of the ground plane as provided by the sea water provides a mirrorima'ge of the antenna so that the antenna 18 appears as having an effective length of one-half wavelength. As shown in FIG. 3, an electrical connection is made between the antenna 18 and the sea water 21 to establish aground plane and also, as will be explained, to permit the high currents appearing at the lower end of the antenna 18, as shown in FIG. 3, to flow into the water. In accordance with teachings of this invention, the length of a resonant antenna may be significantly reduced by providing a ground plane as described above. As a result, significant cost savings may be achieved, while providing an efficient radiating antenna without the substantial loss of power incurred by low-frequency antennas of the prior art.

There is shown in FIG. 2A a suitable electrical schematic for the connection of a radio generator 26 to the antenna 18. In particular, the radio generator 26 is connected through a coupling 24 to ensure the proper impedance match between the-antenna 18 and the radio generator 26. In turn, the radio generator 26 may be connected to the shell of the helicopter 10 as at 28. As shown in FIG. 2A, the coupling 24 is interconnected with the antenna 18 through an insulating bushing 22 within the housing of the helicopter 10. If the insulating bushing 22 were not provided, it may be understood that the radio generator 26 would be shorted out through the shell of the helicopter l0. Couplings such as that illustrated in FIG. 2A'are well-known in the art and may illustratively take the form of a transformer whose ratio of primary-to-secondary windings is selected to provide the desired impedance match. Further, there are well-known inductive and capacitive matching networks such as that described in recognized textbooks; for example, V.L.F. Engineering, published by PergamonPress, which would serve to interconnect the antenna 18 and the radio generator 26. The radio generator 26 is capable of generating a relatively high power, Iow-frequency signal to be applied to the antenna 18; such radio generators 26 are wellknown in the art and may illustratively takethe form of a solid state transmitter amplifier as manufactured by the Assignee of this invention.

The antenna system as shown in FIG. 2B is similar to that illustrated in FIG. 2A,'with the exception that a counterpoise 32is deployed from the heilcopter 10. As shown in FIG. 2B, the leading end of the counterpoise 32 is electrically connected to the radio generator 26, by the housing of the helicopter 10 to which is connectedat point 30, the counterpoise 32. As will now be explainedwith regard to FIG. 3, the counterpoise 32 serves toreduce the length required of the antenna 18, as well as to reduce the voltage gradients established about the helicopter 10. As indicated above, if it were desired to propagate a 22 KHz radio signal, a half wavelength antenna of the prior art would have a length of approximately 22,000 feet; in contrast, an antenna 18 having a ground plane in accordance with the teachings of this invention would require an approximate length of 1 1,000 feet. To still further reduce the length of the antenna 18, the counterpoise 32 is deployed, whose length may be effectively 'substracted from that of the antenna 18. The length of the counterpoise 32 is 'limited, however,.by several considerations. First, the vertically extending antenna 18 is amore efficient radiator per unit length; as a result, there will be a trade-off in radio wave propagation efficiency and the length that the antenna 18 may be decreased. Further, the use of the counterpoise 32 reduces the voltage gradients established about the housing of the helicopter 10. With regard to FIG. 3, it may be seen that the highest voltage gradients established by the antenna 18 are disposed at its upper-most end. Thus, by inserting the radio generator 26 at point A as opposed to point B, the voltage gra dients established about the helicopter housing are reduced. In effect, the high-voltage gradients are shifted from the helicopter to the end of the counterpoise 32.

To reduce possible corona effects due to the high voltage gradients'established at the end of the counterpoise 32, a sleeve 34 of greater diameter is disposed about the end portion of the counterpoise 32. As illustrated in FIG. 2B, the sleeve 34 may be centered about the counterpoise 32 by a series of spacers 35. The sleeve 34 is made of a suitable, electrically conductive material to increase the surface area across which the high-voltage gradients are established. In one illustrative embodiment, the counterpoise 32 has a length of up to 2,500 feet and the sleeve 34 may be disposed over the last,.approximately 200 feet of the couterpoise 32. lllustratively, the sleeve 34 may have a diameter of 6 to 12 inches thereby to increase effectively the radiating surface area of the counterpoise 32,. The counterpoise 32 may be very similar in construction to the antenna 18, having a diameter of approximately Vs to A inch.

With regard to FIG. 2C, there is shown a further schematic illustration of the manner in which a counterpoise 32A is coupled to the radio generator 26. In this illustrative embodiment, the counterpoise 32A is not connected to the housing of the helicopter housing 10 as shown in FIG. 2B, but rather is connected to the radio generator 26 through an insulating bushing 38 disposed within the housing of the helicopter. ,The

radio generator 26, in turn, is connected through the coupling24 to the housing of the helicopter at point 36. Further, as shown in FIG. 2C, the antenna 18 is also connected at the point 36 to the housing of thehelicopter 10.

In the above discussion, the operation of this invention has been described with respect to sea water. It is posed. Sea or ocean water has audegreeof conductivity due to the presence of salt and other impurities therein sufficient to establish the ground plane for the antenna 18. As a degree of conductivity, i. e., the presence of.

salf within the water, decreases, more power would be required of the generator 26 to compensate for the less-than-ideal ground plane which has been established. A ground plane of sufficient efficiency is established in water having at least -l.0 part per thousand of NaCl, corresponding to a conductivity of 0.1 mhos/meter (based on water at 32F).

With regard to FIG. 4, there isshown an illustrative embodiment-of the drogue 20 in accordance with the teachings of this invention. In particular, the drogue 20 includes a light-weight housing 44 made of adurable, light-weight material, such as wood or plastic, for forming thereina cavity for;receiving water, in a manner that will be explained. The outer surface of the cylindrically shaped drogue 20 is considerably greater than that, of the antenna 18, to provide thereby a relatively large surface for contact with the sea water 21. As a result, an efficient, low-resistance electrical contact is maintained between the antenna 18 and the sea water, and the high currents appearing at the lower portion of the antenna 18 will be conducted into the sea water 21. As shown in FIG. 4, the lower surface portion of the drogue-20 is coated with a low-resistance metal layer 62 or other suitable material vto provide thejrelatively large surface area for contact with the sea water} Further, an electrically conductive path 64. is provided from the layer 62 to theantenna IS. A universal connector 40 is connected between the antenna 18 and an interconnecting support member 42; significantly, an

electrical path is established between the antenna 18 and the layer 62 by a suitable means including the path 64 and the universal connector 40. It may be understood that if the antenna 18 were disposed within the sea water, that a very high current per unit of area measurement would flow into the water, with the resulting generation of heat within the lower portion of the antenna. Dependent upon the power of the radio generator 26 and therefore the magnitude of the current flow, the conductor and/or surrounding water would be heated to relatively high temperatures to melt the conductor and/or release gases in the water. In the latter case, a layer of gas would tend to form about the surface of the antenna 18, with the resultant loss of the low resistance contact between the antenna 18 and the water 21. To minimize the probability of the formation of such a gas layer and to ensure a low-resistance path between the antenna 18 and the water 21, the drogue 20 is provided with a large exposed surface of a material having the properties of low resistance and ofbeing non-corrosive when placed in sea water. Illustrative examples of such material are cooper or treated zinc.

The drogue 20 would also serve to stabilize the position of the bottom end of the antenna in the sea water 21. If the endof the antenna 18 were allowed to be withdrawn repeatedly from the sea water, the efficiency of the radio wave transmission would. be decreased greatly. Thus, thedrogme 20 is provided with a door 54.which maybe moved from a position blocking an opening 56 within the housing 44 to-an open positionwhereby water isintroduced into 'ordischarged from the housing 44; Asjshown in FIG. 4, the door '54 is pivoted upon a suitable mount 58 and'may be opened upon actuation of a solenoid 48 which is connected to the door 54 as by a suitable flexible wire 52 disposed to move freely over a roller 60. The solenoid 48 is actuated selectively by a'signalreceived by an FM receiver disposed within a container 46at the top-most portion of the housing 44.

In an illustrative embodiment of this invention, the axial length of the housing 44 wasmade in the order of l2feet, with a diameteriof' 1% to 2 feet. The layer 62 of an electrically conductive material extends from the bottom of the housing 44 adistance of 6 feet. An electrically conductive layer of the aforementioned dimensions was able to dissipate efficiently the current provided by a-radio generator of 20 kw." i

In operation of the antenna system in accordance with teachings of this invention, the antenna 18 with the drogue 20'attached thereto isdeployed from the helicopter 10 at a relatively low altitude over the water 21. With the drogue 20 disposed in the sea water 21, the water naturally flows intothe opening 56 to open thereby the door 54. As a result, the housing 44 fills with water to add thereby weight and stability to the drogue 20. Thereafter, the helicopter l0 ascends while the winch reelsout'the antenna 18. Depending upon the frequency of the radio wave to be transmitted, the helicoptenascends to anvaltitude above the water 21 corespondinglto approximately one-fourth of the wave length of the operating frequency of that radio wave to be propagated; of .course,'the antenna 18 has been reeled out a length corresponding to. the altitude of the helicopter 10 to form thereby a resonant antenna element whose length is one-fourth that of the radio wave transmission, the helicopter l0 simply flies in circles at a speed of approximately 50 knots, wherebythe drogue 20 remains disposed in a relatively stationary position and the antenna 18-swivels as by the universal connector 40. By circling, as opposed to hovering, a more eff-- cient use of fuel is achieved by the helicopter and a longer transmitting time is obtained. To take up the antenna 18, the helicopter l0 descends as the winch l4.

withdraws the antenna 18. To remove the drogue 20 from the sea water 21, a transmitter (not shown) within the helicopter 10 is actuated to send a signal to the FM receiver within thecontainer 46, which, in turn, serves to energize the solenoid 48. As a result, the armature 50 of the solenoid 48 is withdrawn to open the door 54 and to permit the discharge of the water originally taken into the drogue 20, In this manner, the empty drogue 20 is lifted fromthe water without imposing the additional weight of the trapped water upon the antenna l8 and also upon the helicopter 10.

Thus, there has been shown a method and apparatus for efficiently deploying an antenna 18 of reduced length, for radiating-relatively low frequency waves.

Basically, the reduced length is attributable to the establishment of the sea water as a ground plane for the antenna by a suitable low-resistance contact means such as the described drogue. Further, the length, as well as the potentially interfering high-potential gradients, are reduced by the deployment of a counterpoise from the helicopter.

Numerous changes may be made in the abovedescribed apparatus and the different embodiments of the invention may be made without departing from the spirit thereof; therefore, it is intended that all matter contained in the foregoing description and in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed:

1. Apparatus for propagating relatively low frequency radio waves from an antenna, said antenna adapted to be deployed from an aircraft and connected by an impedance matching coupler to a low-frequency wave generator having a predetermined operating wavelength comprising;

a. a counterpoise deployed from the aircraft and coupled electrically to said generator;

b. an antenna connected at its upper end to said coupler and suspended substantially vertically from the aircraft a length substantially equal to one-fourth of the operating wavelength 'of said low-frequency generator. and

c. means for providing a low-resistance path from a lower end of said antenna to a body' of water to provide thereby a ground plane for said antenna.

2. Apparatus as claimed in v claim 1, wherein said counterpoise includes a sleeve disposed about the remote end of said couterpoise for providing an increased surface area of an electrically conductive material whereby the potential gradients normally established at the end of said counterpoise are reduced.

3. Apparatus as claimed in claim 1, wherein said lowresistance means comprises a drogue, presenting an exposed outer surface of an electrically conductive material, and means for establishing an electrically conductive path from said outer surface to said antenna. I

4. Apparatus as claimed in claim 3, wherein said drogue is constructed to have a greater diameter than that of said antenna to thereby provide a larger outer surface for contact with the body of water.

5. Apparatus as claimed in claim 4, wherein said electrically conductive surface comprises a material having a relatively low resistivity and having the property of being non-corrosive in the presence of sea water.

6. Apparatus as claimed in claim 3, wherein said drogue comprises means for defining an opening in its lower extremity whereby water is introduced and discharged from said drogue.

7. Apparatus as claimed in claim 6, wherein said means for introducing and discharging water comprises a door disposable from a first position wherein said opening is blocked to a second position wherein water flows through said opening.

8. Apparatus. as claimed in claim 7, wherein said drogue includes solenoid means for disposing said door from its first to its second position and radio means responsive to a command signal for actuating said solenoid means to dispose said door from its first to its second position. 

1. Apparatus for propagating relatively low frequency radio waves from an antenna, said antenna adapted to be deployed from an aircraft and connected by an impedance matching coupler to a low-frequency wave generator having a predetermined operating wavelength comprising; a. a counterpoise deployed from the aircraft and coupled electrically to said generator; b. an antenna connected at its upper end to said coupler and suspended substantially vertically from the aircraft a length substantially equal to one-fourth of the operating wavelength of said low-frequency generator, and c. means for providing a low-resistance path from a lower end of said antenna to a body of water to provide thereby a ground plane for said antenna.
 2. Apparatus as claimed in claim 1, wherein said counterpoise includes a sleeve disposed about the remote end of said couterpoise for providing an increased surface area of an electrically conductive material whereby the potential gradients normally established at the end of said counterpoise are reduced.
 3. Apparatus as claimed in claim 1, wherein said low-resistance means comprises a drogue, presenting an exposed outer surface of an electrically conductive material, and means for establishing an electrically conductive path from said outer surface to said antenna.
 4. Apparatus as claimed in claim 3, wherein said drogue is constructed to have a greater diameter than that of said antenna to thereby provide a larger outer surface for contact with the body of water.
 5. Apparatus as claimed in claim 4, wherein said electrically conductive surface comprises a material having a relatively low resistivity and having the property of being non-corrosive in the presence of sea water.
 6. Apparatus as claimed in claim 3, wherein said drogue comprises means for defining an opening in its lower extremity whereby water is introduced and discharged from said drogue.
 7. Apparatus as claimed in claim 6, wherein said means for introducing and discharging water comprises a door disposable from a first position wherein said opening is blocked to a second position wherein water flows through said opening.
 8. Apparatus as claimed in claim 7, wherein said drogue includes solenoid means for disposing said door from its first to its second position and radio means responsive to a command signal for actuating said solenoid means to dispose said door from its first to its second position. 